If a gene is to be analyzed or transferred to another organism, that gene must first be located and isolated. For instance, if we want to transfer a human gene for growth hormone to bacteria, we must first find the human gene that encodes growth hormone and separate it from the 3.2 billion base pairs of human DNA. So far, in our consideration of gene cloning, we’ve glossed over the problem of finding the DNA sequence to be cloned: the solution to this problem has been purposely delayed until now because, paradoxically, we must often clone a gene to find it.

This approach—to clone first and search later—is called “shotgun cloning” because it is like hunting with a shotgun: the pellets spray widely in the general direction of the quarry, with a good chance that one or more of the pellets will hit the intended target. In shotgun cloning, we first clone a large number of DNA fragments, knowing that one or more contains the DNA of interest, and then search for the fragment of interest among the clones.